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I'm new to this forum and hoping for some assistance with my 86 635 CSI.

I have a problem with the heater, in regulating the cabin temperature.

I replaced the heater valve with a used item today since I assumed this was faulty. I still have problems but the heater valve is regulating, just way out of calibration. I'll explain:

I started the car and set the dial to 21C. After a time the cabin reached 26C and was still rising, and the supply air was an incredible 51C.

I turned the dial down to absolute minimum (less than 16C) and the valve was closing, since the supply air temperature dropped, but only to 30C, so still actually heating the cabin.

If I turn the dial up just a fraction, the supply air temperature rises up to 39C, despite the dial setting of only 16C.

So, the valve is regulating, but way out of calibration with the setting on the temperature dial. The cabin is roasting, so I need some advice on this, and if possible an explanation of how the system controls so I can start fault finding.

I also need to know if the 2 pin plug on the heater valve has to be a specific way round. It can be fitted either way so I assume it makes no difference.

I'm pretty competent at electrical testing - any advice would be gratefully received.